1. Field of the Invention
The present invention relates to a draft protection device or wind screen for a precision balance. The draft protection device includes a base and a stationary rear wall connected to the base and a front pane whose top is connected by means of two connecting bars to the rear wall. The draft protection device further includes two sliding doors which form side walls and are slidably mounted in longitudinal guide means in the connecting bars and in the base.
2. Description of the Related Art
Draft protection devices of the above-described type are known in the art. These devices have the purpose to prevent falsifications of the weighing results due to air flows.
EP 234008 discloses a precision balance with a weighing space which surrounds the weighing dish on all sides. The weighing space is formed by a stationary front pane and a sliding door on each side which can be moved rearwardly. In this known precision balance, the weighing mechanism and the electronic system are mounted in a housing behind the weighing space, i.e., behind the rear wall forming the weighing space. The housing is capable of completely receiving and guiding the sliding doors in the rearwardly moved position. Rails supporting the side door are provided in the base of the balance and at the upper side of the balance. The rails extend from the front end to the rearward wall of the balance housing and facilitate a precise guidance of the door along the entire sliding distance. An electric drive for sliding the sliding door acts on a plate which is rearwardly movable together with the doors and which is arranged underneath the bottom wall of the balance. This configuration ensures a problem-free operation.
If the weighing space formed by the sliding doors on the sides, by the rear wall, by the front pane and by an upper cover is placed on a balance with an upper weighing dish with the weighing mechanism being completely received in the base of the balance, the sliding doors are not guided in the area behind the rear wall of the weighing space. An example of such a balance is known from German patent 35 08 873. When the sliding doors on the sides are pushed rearwardly, the lower edges and upper edges of the side walls leave the guide means provided on the draft protection device and are moved completely unguided in a cantilevering manner into the free space behind the balance. The portion of the sliding doors remaining in the guide means is very short, so that the weight of the door, on the one hand, and any force applied for displacement of the door which is not introduced in the center thereof, on the other hand, have the result that the door is tilted rearwardly and, consequently, is canted in the guide means. In order to provide a longer length for the guide means, it is possible, for example, as shown in German patent 35 08 853, to provide a guide rail which projects beyond the rear wall of the balance. However, such a configuration is not only optically unsatisfactory but also practically, and only partially solves the problem.
In addition, U.S. Pat. No. 1,784,098 describes a draft protection device for a precision balance in which the front pane is slidable without lateral guide means toward the top beyond the upper cover of the weighing space. For this purpose, the front pane is longitudinally guided in a very stiff frame which is connected to the side walls and is held in the raised position thereof by means of a counterweight which is connected to the front pane through two belts which travel around two rollers and act on the lower edge of the front pane. In such a balance, if the front pane is not held exactly in the middle at the grip thereof and is moved upwardly in this manner, the pane can cant within the lateral guide means with increasing lifting height.